The present invention relates generally to integrated optical circuits and more specifically integrated optical circuits based on diffractive optical elements mounted on a transparent optical substrate.
It has been proposed that integrated optical circuits could be constructed by mounting reflective planar optical elements on a transparent substrate and coupling the elements by means of internal reflections from the mirrored surface of a transparent substrate. The planar optical elements would direct, focus or otherwise diffract an incident optical signal in a desired manner. This approach would allow complex optical devices to be constructed and interconnected in a planar fashion similar to electrical integrated circuits.
This approach has had limited success because of inherent reflection losses and the difficulty of constructing planar elements that are efficiently coupled to the internal optical signal. Mirrored surfaces of the substrate are commonly constructed by applying a thin film of metal to a transparent substrate, but known thin film materials have losses of several percent, and the signal strength is lost exponentially if multiple reflections are necessary. Further, if the planar optical elements are not in intimate contact with the substrate surface there can be large losses associated with getting the optical signal out of and back into the substrate.
Diffractive optical elements (DOEs) are ideally suited for the reflective planar optical elements since they can be integral with the surface, but they suffer from several deficiencies. They typically require a metallic coating to operate in the reflective mode and this results in the loss of optical signal. If the dimensions of the diffracting objects in the DOE are much larger than the wavelength of light in the substrate they will diffract the light into modes other than the desired mode, which can result in a loss of efficiency and cause undesirable errors such as crosstalk. This problem can be somewhat minimized by using blazed grating patterns in which the objects are shaped to preferentially diffract light in a desired direction. A sawtooth blazed diffraction grating is an example, typically fabricated using a shaped engraving tool. Nevertheless, the three dimensional nature of blazed gratings makes them difficult to fabricate on the surface of an optical substrate, however, diffraction of light into undesired modes and directions is still a problem.
If the dimensions of the diffracting objects approach the wavelength of light, the undesired modes or directions can be minimized or eliminated by the proper selection of incident angles and size of the diffracting objects. Such a device is generally known as a holographic optical element (HOE), which is a subset of DOEs. If these devices are constructed by means of patterning the depth of a reflective surface they are known as surface relief or phase holograms since the different depths of the diffracting surface cause varying phase shifts in the diffracted light. These phase shifts can be adjusted to cause constructive interference of light in the desired direction or mode of the directed light signal by adjusting the depth of the pattern. If the pattern of refracting objects is coated with a reflecting metal film, losses could be as low as a few percent since very little of the light energy is absorbed in such a device. In practice, however, it is difficult to construct such a device on the surface of a substrate in a manner that exhibits high efficiency and can be efficiently manufactured. Electron beams can directly write patterns of these dimensions onto a substrate but this is a very slow and expensive process and does not lend itself to producing the surface relief required for a phase type hologram. Embossing is used to reproduce surface type holograms on transparent plastics (e.g. credit card security holograms) but the tolerances and stability of these materials are not suitable for most applications.
On the other hand, volume holograms can also be created by exposing a photographic emulsion to a pattern of interfering laser light. A pattern of diffracting objects is created within the volume of the emulsion. HOEs constructed with this method can have high efficiency, but they are notoriously difficult to produce and are subject to deterioration due to environmental effects.
As would be evident from the above problems, there is a need for a method of forming an optical integrated circuit based on diffractive optical elements on the surface of a transparent substrate with high optical efficiency that can be mass-produced at a relatively affordable cost.
In one embodiment, provided is an integrated optical device having an optical substrate, wherein an incident light signal is propagating within the substrate in a primary direction of propagation reflecting off a top surface of the substrate under total internal reflection. The integrated optical device also has a diffractive optical element having a plurality of spaced-apart members formed of an optically transparent material and that are disposed above the top surface of the substrate such that the incident light signal is reflected within the substrate along a desired direction of propagation.
In accordance with an even further embodiment, provided is a diffraction grating for use with an optically transparent substrate and having a plurality of members formed of a second optically transparent material and disposed on a top surface of the substrate. The members are spaced apart a spacing distance and have member widths. The sum of the spacing distance and the member width is chosen such that a light signal traveling within the substrate under total internal reflection off the top surface in an incident direction of propagation and incident upon the diffraction grating is reflected into a first diffracted order propagating within the substrate in a reflected direction of propagation. The reflected direction of propagation defines an angle with respect to the incident direction of propagation and the reflected light signal is propagating within the substrate under total internal reflection.
In accordance with another embodiment, provided is a method of routing an incident light signal. The method includes a step of transmitting the incident light signal in an optical substrate under total internal reflection off of a top surface of the substrate. Performed in another step of the method is a step of disposing a plurality of spaced-apart strips above the top surface of the substrate for receiving a portion of the incident light signal. The strips are disposed such that the strips form a diffraction grating that reflects the incident light into a first diffracted order propagating within the substrate in a reflected direction of propagation defining an angle with respect to an incident direction of propagation and propagating within the substrate under total internal reflection.
In accordance with another embodiment, provided is an integrated optical device having a substrate formed of an optically transparent material and having a light signal traveling within the substrate under total internal reflection. The integrated optical device also has a first diffractive optical element formed of a first plurality of spaced-apart members disposed above a top surface of the substrate so as to reflect the light signal within the substrate in a desired direction of propagation. Furthermore, the integrated optical device has a second diffractive optical element formed of a second plurality of spaced-apart members and disposed above the top surface of the substrate to receive the reflected light signal from the first diffractive optical element and disposed to output the reflected light signal for propagation within the substrate.